The recent growth of the Internet has created both a tremendous demand for additional subscriber access to public switched telephone networks (PSTN) and a demand for additional bandwidth for the access. The former demand is being met by deploying additional analog access lines so that modems can be used for the Internet access, and the latter demand is being met by deploying integrated service digital network (ISDN) lines over twisted cables. In the first case, although Internet access is provided, it is not of sufficient bandwidth to provide the graphic-rich service which typically is desired. In the latter case, a subscriber's main standard plain old telephone service (POTS) line is not converted to ISDN in order to maintain the subscriber's POTS access in the event of a local power outage or electronic system failure because the ISDN service is dependent upon the residence power and proper function of the ISDN electronics. In both cases, the installation of an additional copper-pair based access line is expensive and time consuming due to a general shortage of pre-existing installed cable pairs.
In the latter case, the ISDN installation on existing cable pairs is limited to about 70% of the installed base due to the manner in which ISDN transport was designed specifically for non-loaded cable plants. The existing copper cable outside plant was constructed in accordance with design requirements specifying that for local loops exceeding 13 kilo-ohms (k.OMEGA.), or approximately 18 kilo-feet (kft) which is equivalent to 5,486 meters, loading coils or filter capacitors are added to remove voice frequencies shifted above 4 kilo-Hertz (kHz) due to the loop resistance. The REA loop survey of 1986 indicates that for the US as a whole, approximately 85% of all loops are non-loaded. Since ISDN uses a digital signal operating at a center frequency of 40 kHz, it will not transmit in the presence of a load coil. Bridged taps or branches attached to a primary cable run further reduce the reach of an ISDN signal, with the net result being that only about 70% of all existing subscribers can have ISDN service added without additional construction expenses, as reported by Pacific Bell in early 1996. Therefore, providing ubiquitous digital access for all telephone subscribers is limited by both the number of pre-existing cable pairs and limitations imposed by the design of the telephone outside plant.
One solution is for the telephone company to simply install more copper cables. In fact, record amounts of copper cables are being installed in response to the huge demand for added lines. But this is not a financially viable alternative for the telephone companies due to the long depreciation schedule for these cables. It is generally recognized that a higher-bandwidth medium, such as fiber optic cable, is the ultimate solution for the digital access though the fiber connectivity precludes the lifeline access in the event of local power failures. While the technical and financial issues related to fiber installation are being worked out, installing copper cables only consumes capital and delays the day for fiberization.
One other approach is to more efficiently utilize existing phone lines for high-speed digital transmissions. The phone lines are made of twisted copper pairs and are configured in a star-like architecture that is suitable for bi-directional communications. The principal technology for placing a digital signal onto a copper pair that originally provided only analog dial tone is called integrated service digital network (ISDN). ISDN was developed in the 1980's, when the state-of-the-art digital encoding technology resulted in the standards as described in Bellcore documents TR-TSY-000393 and TR-TSY-000397. The basic transmission speed, called BRI for ISDN is 160 kilobits per second (kbps). This digital rate and its corresponding communication method are digital subscriber line (DSL). It is significant that ISDN was designed specifically for a non-loaded telephone plant since loading capacitors effectively attenuated high frequency digital signals. The non-loaded cable plant reaches 18 kft but only 85% of all subscribers on average. This results in a problem with respect to reaching all subscribers desiring ISDN services. Since 1990, the development of microprocessors has significantly improved the performance of communication chipsets. High bit rate subscriber line (HDSL) chipsets can run at 784 kbps or even 1 Mbps to transport one half of a T1/E1 digital loop carrier signal in an application called "Repeaterless T1/E1." Other types of high speed communication technologies for the twisted pairs, such as asymmetric DSL (ADSL), are emerging from labs but are still too expensive for wide range applications. HDSL technology can be used to transport either one high speed signal or several lower speed signals through multiplexing and demultiplexing. Installing one high bit line for multiple lower bit signals is more cost effective than installing several lower bit lines. This approach was explored by several inventors in the past.
By way of example, Carse et al., U.S. Pat. No. 4,730,311 describe a multiplexer for use in a telephone system in which a plurality of subscriber locations are connected to a central office by a single subscriber loop. Carse et al. focus on the design of the multiplexer rather than the entire communication system. Their technique applies generally to any methods of digital transmission, consequently the transmission rate is arbitrary. The subscribers are defined to be locally powered and backed-up with battery power. The battery back-up can only last for a limited period of time in the case of local power loss. For the design of the multiplexer, Carse et al. do not define either a digital interface or standard of loopback testing. Also, the configuration of the central office is not described.
Litteral et al., U.S. Pat. No. 5,247,347 and Coddington et al., U.S. Pat. No. 5,410,343 define how to provide digital video signals from a video information provider to one or more of a plurality of subscriber premises. However, the multiplexers used in both systems mainly perform frequency domain multiplexing/demultiplexing which is inherently disadvantageous with respect to time domain multiplexing/demultiplexing. The power source of the multiplexers is not specified. In addition, Litteral et al. and Coddington et al. only describe transport and encoding of specific video signals rather than generic digital signals. Bliven, U.S. Pat. No. 5,459,729 describes a method and apparatus for transmitting and receiving multiple telephone signals over a single twisted pair. Two conventional telephone signals are converted into one digital signal and then transported over a single twisted pair at a rate of 160 kbps. Creating a multiplicity of telephone channels in this way is sufficient for analog POTS but is too low to provide adequate Internet access.
Accordingly, it is a primary object of the present invention to provide a communication system that transports multiple ISDN signals and a POTS channel over a single twisted cable pair at a high bit rate. It is a further object of the invention to provide line powering to a remote terminal to avoid dependence upon local power and to provide for a metallic POTS access in the event of electronic failures. This invention is subsequently referred to as a "multiple ISDN and POTS carrier system" or abbreviated as "MIPCS."